The most common and most likely answer to this crossword clue is the 5 plus 3 letter word FLASH MOB
I've seen this crossword clue in Financial Times
F
L
A
S
H
M
O
B
More clues leading to the result FLASH MOB
- Unexpected gathering of streakers?
- Two seconds before, British crowd suddenly assembled
- Fool in fat group coordinated online
- Expose yourself to the mafia in brief public assembly
- Ad hoc social media group: “keep it quiet; Farah’s putting on weight”
- Vulgar book about second public gathering
- Cooked one third of shanksfor group formed online
- Quiet doctor surrounded by fat people suddenly gathering
- American jerk among fat group making a scene in public
Buddy explains!
The straight part
We know that "Ad hoc social media group" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ad hoc social media group = flash mob.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.